The present invention relates to the determining of angles-of-arrival of incoming RF signals with an antenna array of RF sensing elements. More particularly, the present invention relates to determining angles-of-arrival and the polarization of incoming RF signals as sensed by an antenna configuration in which antenna elements are mounted onto a non-planar surface in a fashion whereby the elements conform to the surface. This RF signal sensing array is also known as a conformal array of antenna elements. It is in such a non-planar array that the antenna elements of the conformal array are, by necessity, of differing, yet known, orientations of polarization.
The object of the present invention is to improve upon existing and well established interferometric techniques which apply measurements of phase differences between sets of antenna element pairs of similar orientations of their respective polarizations to determine unambiguous angles-of-arrival of incoming RF signals. The improvement manifests itself in the exploitation of amplitude measurements or the equivalent powers of incoming RF signals, which when processed according to the teachings of this invention, provide the required polarization induced phase correction allowing for the determination of the angles-of-arrival and the polarization of incoming RF signals with antenna elements of diverse orientations of polarization without the necessary application of iterative, linearly approximated solutions.
Conventional interferometry is a method reliant upon the measurement and processing of the electrical phase differences between the signals received at the ports of a number of antenna element pairs to derive the hitherto unknown direction of arrival in angle space of a received RF signal.
In the prior art of conventional interferometry, any polarization mismatch that exists between the set of receiving antennas and the polarization state of an incoming radar signal has no effect on the relative electrical phase of the voltages induced at the various antenna element terminals. While the polarization mismatch affects both the phase and the amplitude of the various voltages, it does so substantially identically, provided the polarization of the receiving antennas are all substantially equivalent. Thus, in conventional interferometry the phase difference, .DELTA..PSI., between any pair of antennas is solely a function of the path difference traveled by the incoming wave-front as it impinges upon the particular antenna element pair. This relationship is expressed as EQU .DELTA..PSI.=2.pi.d/.lambda.sin(.THETA.)-2n.pi. 1!
where the baseline, d, is the distances between the phase centers of the two antenna elements and .lambda. is the wavelength of the received radar signal. The angle .THETA. represents the unknown angle between the geometric normal to the baseline d and the direction of the incoming wave.
Through operational inversions of Equation 1, the angle .THETA. can be obtained in terms of the measured electrical phase difference .DELTA..PSI. provided that a possible angular ambiguity, represented here by the term 2n.pi., can be removed. This ambiguity to be resolved exists whenever the ratio d/.lambda. is larger than one-half.
Since the direction of an incoming wave is uniquely defined in space by two polar angles, two equations in the inverted form of Equation 1 using non-parallel baselines are required to obtain these angles-of-arrival. However, the removal of ambiguities may require additional pairs of antennas. Methods for removing these ambiguities are well known. One of several well known methods for resolving the angles-of-arrival uses a series of two-channel interferometers with progressively increasing separations such that the lowest separation is equal to .lambda./2 at the highest operating frequency. The first angles-of-arrival estimate is therefore the coarsest but is unambiguous. Using the remaining larger baseline interferometers subsequently refines this angles-of-arrival measurement. Another well-known method uses three or more interferometers with baselines greater than .lambda./2. However, the antenna separations are selected in specific ratios such that ideally, and in the absence of noise, only one pair of angles-of-arrival values is consistent with the measured phase differences and the associated ambiguity numbers.
Referring now to the prior art figures in detail wherein like reference numerals indicate like elements throughout the several views, the prior art can be illustrated by a flowchart of the process in FIG. 1 and a block diagram of the apparatus in FIG. 2. RF sensing elements of similar orientation of polarization 101 are selected by an RF selection network 102 so that receivers 104 may receive their respective voltages 103 and measure the signal frequency and the inter-antenna phase differences and then convert the analog measurements to digital form. The measured digitized phase differences 105 and measured digitized signal frequency 106 are sent to a digital interface 107 of a digital signal processor designated 108. Any polarization diversity among the individual antenna elements, intentional or otherwise, will produce errors in the respective phase differences of 105. These angular errors, should they arise, are not first accounted for and then corrected for in the prior art. Within the digital signal processor, the digitized phase differences 110 and the measured digitized signal frequency 109 in the reciprocal form of wavelength are applied to the resolution of angular ambiguities 113. The resolved unambiguous phase differences 114 and the wavelength 115 are applied to the determination of angles-of-arrival 116.
FIG. 2 illustrates a block diagram of a prior art apparatus. Individual RF sensing elements individually identified as 101 are mounted on a surface without curvature 202. Each element is connected to an element selection network 204 by an electrically conductive line 203. Each antenna element generates a voltage when excited by a RF wave-form. The element selection network selects pairs of antenna elements for the application of interferometry. At least two receiver channels 206 are required. The resulting antenna pairs sensed voltages are conveyed along a conductive harness 103 to a RF receiver 206. For each signal pair difference the individual receiver outputs a phase difference 207. Additionally, the receiver will output a measure of frequency of the RF signal 208. The phase differences 207 and frequency 208 of the RF signal are input signals to a special purpose computer 209.